Add client-based NodeSet2 export functionality

[Copilot]

Proposed changes

Enables OPC UA clients to export browsed address space nodes to NodeSet2 XML format. Previously, NodeSet export was only available server-side via NodeStateCollection.

Implementation

Core Export API (CoreClientUtils.NodeSetExport.cs)

• ExportNodesToNodeSet2(ISystemContext, IList<INode>, Stream) - exports client nodes to NodeSet2 XML
• Converts INode instances to NodeState representations for all node classes
• Preserves attributes, references, and namespace mappings

Client Integration (ClientSamples.cs)

• ExportNodesToNodeSet2(ISession, IList<INode>, string) - convenience wrapper for file export

Usage

// Browse and collect nodes
var nodes = await clientSamples.FetchAllNodesNodeCacheAsync(
    uaClient, ObjectIds.ObjectsFolder, 
    fetchTree: true, filterUATypes: true);

// Export to NodeSet2
clientSamples.ExportNodesToNodeSet2(session, nodes, "address-space.xml");

Related Issues

N/A

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

Leverages existing UANodeSet.Export() and NodeStateCollection.SaveAsNodeSet2() infrastructure. Exported NodeSet2 files are compatible with existing import mechanisms.

Tests cover basic export, multiple node types, and round-trip import/export validation.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Client based nodeset export</issue_title>
<issue_description>### Type of issue

• Bug
• Enhancement
• Compliance
• Question
• Help wanted

Using the OPC.Ua.Client library is it possible to connect to a server and export the address space to nodeset file? I am aware of the NodeStateCollection class which can be used on the server but i am intrested in a client based solution.

</issue_description>

<agent_instructions>Please implement this Feature </agent_instructions>

Comments on the Issue (you are @copilot in this section)

@mregen This is currently investigated for a next release... The console client sample already contains code to browse and import a complete address space of a server. Only the the Nodeset2 export is missing.

• Fixes Client based nodeset export #1864

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